The operation of such a transistor is based upon the movement of minority carriers (electrons in the case of an NPN transistor) which are injected from the emitter into the base upon the application of a saturation voltage to the latter. At the instant of cutoff, therefore, the potential difference between the collector and the emitter may exceed the supply voltage which may give rise to problems. Thus, upon termination of the state of saturation in the presence of a cutoff voltage it becomes important to reduce as far as possible the concentration of minority carriers in the base region to minimize the power dissipation resulting from that potential difference.
The delay between the application of the cutoff voltage and the return of the transistor to its normal nonconductive state may be divided into two phases referred to as "storage time" and "fall time". The storage time involves the sweep-out of mobile charge carriers from the collector region whereas the fall time measures the normalization of the concentration of minority carriers in the base region. These two phases are somewhat interrelated.